The invention relates to a laser module, in particular for coupling to an optical fiber, having a semiconductor laser in a housing and having an isolator.
Such laser modules are disclosed in EP 02 59 888 B1 and DE 42 32 327 A1. Both publications describe a laser on a chip carrier whose light is collimated using a first lens and penetrates the housing wall via an obliquely positioned flat aperture. The housing is sealed hermetically tight in this case by the flat aperture. Outside the housing, the light is focused onto the end face of an optical fiber by a second lens. Additionally described in the second publication is an isolator which is seated outside the hermetically enclosed volume in a stub which projects into the housing and carries the aperture at its end.
DE 44 22 322.6 discloses a laser module having a silicon substrate which is micromechanically structured by means of an anisotropic etching technique. A first lens and an isolator are inserted into a first groove. A monitor diode is located in a second groove. The semiconductor laser is mounted on the web between the grooves.
If a laser diode is to be used for very high transmission rates or bit rates, it is necessary to keep retroreflected radiation away from the laser diode. The retroreflections can be produced both at the coupling optical system required for the optical coupling between the laser diode and the fiber, and on the fiber link itself (plug-in contacts, splices). The reflections at the coupling optical system are avoided according to the prior art by providing that the optical surfaces are coated with an antireflection coating and/or installed obliquely with respect to the beam path. The retroreflections from the fiber link cannot be eliminated by these measures. According to the prior art, an optical isolator is used for this purpose. Such an optical isolator consists of a first polarizer, an optical crystal having the Faraday effect in a static magnetic field which rotates the plane of polarization of the light by 45.degree., and of a second polarizer, whose direction of polarization is rotated by 45.degree. with respect to that of the first polarizer. In the forward direction, after rotating by 45.degree. with its plane of polarization parallel to the direction of polarization of the second polarizer, the light linearly polarized in the first polarizer impinges on said second polarizer and is therefore virtually not attenuated. In the opposite direction, the reverse direction, the rotation by 45.degree. has the effect that the light linearly polarized by the second polarizer impinges on the first polarizer rotated by 90.degree. with respect to the direction of polarization of the latter, and is therefore blocked. The optical paths (dotted arrows) and the directions or rotations of polarization (continuous arrows, the angle of polarization having been projected into the plane of the drawing in each case) are illustrated in FIG. 1 for the forward direction (upper row of arrows) and reverse direction (lower row of arrows). In order to achieve low forward attenuation and high reverse attenuation which are important for use, in addition to a high quality of the polarizers and of the Faraday rotator there is a need for a very exact azimuthal alignment of these components relative to one another. In order to protect an isolator against environmental influences, it is necessary, according to the prior art, for all the optical components of the isolator to be jointly sealed hermetically tight in a cylindrical metal housing. The overall size of the entire isolator must be so small in this case that it fits into the microoptical beam path required for the laser coupling. According to the prior art, an outside diameter of 3 mm and a length of 4 mm are customary for this purpose. In the case of mounting the finished isolator in a laser transmitter module, it is necessary to carry out exact azimuthal alignment of the isolator relative to the plane of polarization of the laser diode. This means an additional adjusting step in mounting the laser.